HARDNESS 31 



explained above, it may be fixed at H = 7|, 7^, or 7f according as its hardness approximates 

 more nearly to that of quartz, lies midway between that of quartz and topaz, or approaches 

 more nearly to that of topaz. 



It is sometimes sufficient to determine the hardness of a stone approximately, and, 

 m such cases, when the exact degree of hardness is not required, the scale of hardness need 

 not be used. The softest mineral, talc (No. 1 of the scale), is greasy to the touch. No. 2 

 on the scale, namely gypsum, can be easily scratched with the finger-nail, but this is 

 impossible in the case of calcite (No. 3). A knife scratches calcite easily, fluor-spar (No. 4) 

 less easily, apatite (No. 5) still less easily, and felsjmr (No. 6) only with difliculty, while 

 quartz (No. 7) cannot be scratched at all with a knife. Quartz and members higher in the 

 scale will strike fire with steel more or less readily, while felspar (No. 6) only does this 

 with difficulty and to a small extent, and lower members of the scale not at all. Minerals 

 harder than apatite (No. 5) are capable of scratching ordinary window-glass, and this 

 substance may be fixed approximately at No. 5 in the scale of hardness. The more a mineral 

 exceeds apatite in hardness the more easily will it scratch glass. 



Minerals used as precious stones have the highest degree of hardness. The hardness of 

 the most valuable corresponds to Nos. 10, 9, and 8 on the scale ; those of less value have 

 a hardness denoted by 7, rarely lower than this. Hardness above that of quartz is therefore 

 known as gem-hardness ; a mineral below this standard has little application as a gem, since 

 it will be readily scratched even by dust. Among other constituents, dust contains minute 

 mineral particles especially of quartz, and in cleaning a stone of hardness less than 7 by 

 rubbing it with a cloth it will be scratched by these particles of quartz, and in course 

 of time lose its beauty, becoming dull, rough, and lustreless. Stones of gem-hardness are 

 not so scratched and damaged. Moreover, if stones of different hardness are allowed to rub 

 against each other, as may easily happen if several mounted gems are kept together loosely 

 in a box, then the harder stones will scratch and damage the softer ones. Since diamonds 

 are usually represented in collections of jewels, all other stones, including ruby and sapphire, 

 being softer, are liable to suffer if due care in this respect be not taken. 



With few exceptions, therefore, precious stones possess a high degree of hardness, and 

 very nearly all are capable of scratching glass, a substance usually conveniently at hand as 

 window-glass or as a watch-glass. Since glass naturally will not scratch glass, a genuine 

 precious stone may be easily distinguished from its imitation in glass by the test of hardness. 

 As an aid to the identification of a stone, its position in the scale of hardness should be 

 determined as described above. In the case of a cut stone, however, the process must be 

 reversed to avoid damage to the stone ; that is, the scratching power of the cut stone must 

 be tried upon the scale minerals, commencing with the lower members, until the hardest the 

 stone is capable of scratching is found. In such cases only an approximate value of the 

 hardness can be arrived at, but it will usually be sufficient. 



For such purposes there is no need to use the complete scale of hardness ; the softest as 

 well as the hardest members may be omitted. As a standard for the fifth degree of hardness 

 a small plate of glass serves excellently, and is more easily obtainable than a good piece of 

 apatite. There will be required in addition pieces of felspar, quartz, and topaz. The 

 quartz should be a colourless transparent crystal (rock-crystal) ; the topaz should have a 

 smooth, cleavage surface, so that the slightest scratch may be easily and surely recognised 

 whenever the surface is examined with a lens. For the practical determination of gems the 

 use of any other than these four members of the scale of hardness is superfluous. The softer 

 precious stones, with a hardness less than that of apatite, will be recognised by their 

 incapability of scratching glass ; combined with other easily observed characters, this will 



